1. Field of the Invention
The present invention relates to gas bearings, and more specifically to a gas bearing assembly wherein an inner ram is supported by a plurality of opposed gas bearings from an outer housing for linear movement relative to the housing.
2. Description of the Prior Art
Gas bearings are often used when precise movement of a component is desired, or when motion vibrations must be kept to a minimum. The gas usually used for such bearings is compressed air. Prior art air bearing linear slides have typically taken the form of a base beam upon which a saddle rides, with an air bearing being created therebetween to support the saddle upon and about the beam. Air bearing linear slides of this type are illustrated by those manufactured by Professional Instruments Company of Minneapolis, Minn., Dover Instrument Corp. of Westboro, Mass. and Pneumo Precision, Inc. of Keene, N.H. Such a linear air bearing slide is also shown in U.S. Pat. No. 4,448,460, granted to Yamamoto on May 15, 1984.
This type of linear air bearing slide presents some disadvantages for various applications. For example, the extent of movement of the saddle is limited by having to have a beam directly underneath it. In addition, such linear slides are typically only useful for horizontal applications. If a precise location probe is to be mounted on the saddle, the chance for error location increases as the probe is moved laterally further from the precise sliding and support surfaces of the beam. Pressurized air is usually fed to the saddle portion of such units, and thus movement of the saddle requires that air hoses must follow it to provide air any may bias such movement.
U.S. Pat. No. 3,578,827, granted to Smith on May 18, 1971 does show an air bearing arrangement wherein the beam is movable rather than a saddle. In this case, the pressurized air bearings are supported on a stationary frame. The arrangement shown in Smith, however, does not address many of the problems and concerns mentioned above. In addition, when the beam is moved relative to a stationary frame, it can be extended some distance from the frame. This can cause the beam to droop relative to the housing and a desired axis and thus create errors in precise position location proximate the end of the beam. Some means to counteract such drooping of extended beams in a linear air bearing is thus required in order to maintain precise position location functions.
The manufacture of precision gas bearings in general also presents problems. For gas bearings to be useful for precise position location, a high repeatablity is desired for the movable element (either saddle or beam) when it is moved and then returned to its original location. Such precision is dependent not only on the means used for moving the movable element (e.g., motor, actuator, etc.), but also on the precise mating of the relative sliding surfaces of the movable element and the stationary element. Several United States patents are illustrative of attempts to address these concerns. U.S. Pat. No. 3,578,827, granted to Smith on May 18, 1971 shows the use of springs to load one opposed parallel bearing surface against a precise "home" surface on a beam. U.S. Pat. No. 3,447,840, granted to Gress, Jr. on June 3, 1969 shows the use of a plate of glass as one of the bearing surfaces in an effort to obtain uniform support and location. U.S. Pat. No. 3,272,568, granted to Koorneef et al. on Sept. 13, 1966 introduces the use of magnets to urge the bearing surfaces of the relative movable members together across an air cushion and thereby pre-stress those surfaces to maintain a gas cushion of constant thickness between them.